It is known that biodegradable resins are readily susceptible to degradation in water and soil without producing harmful substances. In consequence, the biodegradable resins have been worldwide noticed from the viewpoint of environmental protection such as waste disposal treatments. Among them, biodegradable resins other than polylactic acid-based polymers exhibit physical properties close to polyethylene. Therefore, it is expected that a film obtained by molding such biodegradable resins will be used in the applications such as agricultural materials, civil engineering materials, vegetation materials and packaging materials in future (for example, refer to Patent Documents 1 and 2).
However, any of the conventional biodegradable films is insufficient in tear strength, in particular, in a machine direction (stretch direction) of the films, and therefore have problems upon practical use.
On the other hand, construction of a recycling-oriented society by change from exhaustible resources to recyclable resources has been noticed. For this reason, as a raw material of the films, there is an increasing interest in not synthetic materials produced from petroleum but materials derived from natural substances. At present, as the materials derived from natural substances, starches have been practically used.
As the starches imparted with film-forming property and physical properties, there have been proposed an esterified vinyl ester graft polymerized starch (Patent Document 3), starch esters (Patent Document 4), and an alloy of a polyester graft-polymerized starch and a polyester (Patent Document 5). Further, it might be considered that by highly modifying starches, the resulting modified starches are further enhanced in film-forming property and physical properties. However, such modification of the starches is unpractical in view of costs.
Also, there has been proposed the method of preparing a composite material of a gelatinized starch and a thermoplastic resin (for example, refer to Patent Documents 6 and 7). Further, there have been various proposals concerning systems to which modified starches are added (for example, refer to Patent Documents 8 to 11).
However, any of these conventional compositions tends to be insufficient in fluidity upon heat-melting. Therefore, although it is possible to some extent to form the compositions into a molded article having a simple shape, e.g., a thick sheet, etc., by an extrusion-molding method, it has been difficult to obtain a molded article having a complicated shape, e.g., a thin film therefrom, by an injection-molding method. Even if the thin film is formed from the compositions, the resulting film tends to fail to have practical physical properties. In addition, since it is required to conduct a gelatinization step and a blending step of the starches separately from each other, there tend to arise the problems concerning production costs.
To solve these problems, there has been proposed the method of providing a composition of an oxidized gelatinized starch and a biodegradable resin (Patent Document 12). In this method in which the gelatinization and oxidation steps are conducted at the same time, since the gelatinization step is carried out in the presence of water and a plasticizer, it tends to be difficult to control decomposition of the starch by an oxidizing agent or blend the aforementioned water, plasticizer, oxidizing agent and resin with each other to a sufficient extent, and therefore there tends to arise such a problem that production costs are virtually increased. More specifically, when the gelatinization, oxidation and compounding steps are conducted at the same time, the resulting biodegradable resin tends to have a reduced molecular weight, and therefore it is difficult to enhance a film-forming property and physical properties of the resin composition.
As illustrated, in the case where the oxidized gelatinized starch pellets and the biodegradable resin pellets are dry-blended with each other upon molding, the blended resin mixture may be subjected to injection molding without any significant problem. However, if the resin mixture is formed into a thin film using a melting extruder ordinarily used in an inflation molding method, kneading of the resin mixture tends to be insufficient, so that there tend to be caused the problems concerning a moldability and physical properties of the resulting resin composition. In addition, in Patent Document 12, since a peroxide is used as the oxidizing agent, the gelatinized starch and the biodegradable resin tend to be insufficient in compatibility therebetween, so that the obtained resin composition may fail to be sufficiently improved in moldability when forming the composition into a film.
In the Patent Documents 13 to 15 cited herein, there are disclosed resin compositions constituted of an oxidized starch and a biodegradable resin which are improved in moldability and mechanical properties, as well as physical properties of compositions obtained by further adding additives and an aliphatic aromatic polyester to the resin compositions. However, any of the above conventional methods fails to improve a film-forming property of the resin compositions and a heat-sealing property of the obtained films when adding a lactic acid-based polymer essentially having a high melting point to the compositions.